With the type of optical smoke detector in which smoke scatters light from an exciter lamp to a photocell there is a choice of disposing lamp and cell so that light is scattered backward or forward from the smoke as in Kompelien, U.S. Pat. No. 3,185,975. Forward scatter disposition has the advantage that particles scatter more incident light forward than backward, but on the other hand requires that light from the lamp must be masked from direct paths to the photocell, as in Hall, U.S. Pat. No. 3,409,885. The masking means hitherto has absorbed a substantial amount of the source light (as is recognized in Tresch, U.S. Pat. No. 4,181,439). This involves an energy loss of considerable importance in battery powered smoke detectors widely used in residences.
It is therefore an object of the present invention to provide a forward scatter type of smoke or other particle detector which conserves light energy.